How your muscles behave like a car.

     In your car you have sensors that measure fluid temperature, fluid viscosity, air temperature, position of certain components. These sensors are put in place to enable certain components of the car or to protect certain components of the vehicle. 

Muscles work the same way.

    Muscles have sensors that measure changes in muscle length, and tension. In the fitness world we call them proprioceptors and mechanoreceptors. We will look at two specific ones: Muscle spindles and Golgi Tendon Organs (GTO's).

Muscle Spindles

    Muscle spindles are located within muscle fibers running parallel to extrafusal muscle fibers. Since muscle spindles are located within muscle fibers or intrafusal muscle fibers. If I may keep with the car metaphor your thermostat is located within the cooling system and measure changes in temperature. Muscle Spindles are located within muscle fibers that measure changes in muscle length. 

Muscle spindles located within muscle fibers that measure changes in muscle length

    The purpose of a thermostat is to protect the engine from overheating. At a certain temperature the flow of coolant changes directions to prevent the engine. Muscle spindles sense a rapid change in muscle length and take measures to protect the muscle from damage. How does it do this? glad you asked.

    During a rapid stretch of the muscle the muscle spindle will send a message via a sensory neuron to the spine. The motor neuron then causes a muscle action of the previously stretched extrafusal muscle fiber. This entire process is called the stretch reflex. Think back to our thermostat metaphor. Engine reaches a certain temperature-coolant is redirected-signals are sent reading the temperature to the dashboard providing you with temperature information. 

    How to muscle spindles apply to exercise? During stretching. I should be more specific-static stretching in particular. When engaged the muscle spindle prevents the muscle from lengthen to the point of injury. A slow stretch (static stretching) then muscle spindles are not engaged thus meaning the muscle can stretch to new plasticity (greater length of muscle after stretching) or in laymen's terms gains in stretching. 

    Another sensor is called the Golgi Tendon Organs or (GTO for short). The GTO is located where the tendon and the muscle connect, and it reads changes to muscle tension. When too much muscle tension occurs then the GTO is engaged and sends a signal to shut it down. It protects the muscle against itself. In our car themed post this is a circuit breaker. Circuit breakers are designed to protect a circuit in case too much voltage goes through a circuit. The breaker will trip and requires resetting. While you can't "reset" a muscle it does have properties associated with a circuit breaker.     

    When the GTO is engaged the muscle is relaxed. This can happen in two ways: autogenic inhibition and reciprocal inhibition. Autogenic inhibition is where the muscle relaxation occurs in the same muscle experiencing tension. Sounds fishy right? lemme explain. During a set of heavy back squat your quads are doing the brunt of the work the adjacent tendon feels the additional tension in the quads, and it relaxes. Reciprocal inhibition works opposite. The opposing muscle group experiences the relaxation. Think of a baseball pitcher winding up before a pitch. The moment when they wind their arm back (rapid stretch and tension) the opposite muscle group in the back of the shoulder relaxes.